Does Reduction-Induced Isomerization of a Uranium(III) Aryl Complex Proceed via C–H Oxidative Addition and Reductive Elimination across the Uranium(II/IV) Redox Couple?

Abstract

Reaction of the uranium(III) bis(amidinate) aryl complex {TerphC(N^<i>i</i>Pr)₂}₂U(Terph) (<b>2</b>, where Terph = 4,4″-di-<i>tert</i>-butyl-<i>m</i>-terphenyl-2'-yl) with a strong reductant enabled isolation of isomeric uranium(III) bis(amidinate) aryl product {TerphC(N^<i>i</i>Pr)₂}₂U(Terph*) (<b>3</b>, where Terph* = 4,4″-di-<i>tert</i>-butyl-<i>m</i>-terphenyl-4'-yl). In terms of connectivity, <b>3</b> differs from <b>2</b> only in the positions of the U-C and C-H bonds on the central aryl ring of the <i>m</i>-terphenyl-based ligand. A deuterium labeling study ruled out mechanisms for this isomerization involving intermolecular abstraction or deprotonation of the ligand C-H bonds activated during the reaction. Due to the complexity of this rapid, heterogeneous reaction, experimental studies could not further distinguish between two different intramolecular C-H activation mechanisms. However, high-level computational studies were consistent with a mechanism that included two sets of unimolecular, mononuclear C-H oxidative addition and reductive elimination steps involving uranium(II/IV).